The present invention relates to an improved photoelectric colorimeter of the type capable of determining the amounts of X, Y and Z primaries or thristimulus values in a color sample.
It is to be understood that the term "primary" as employed herein is intended to convey that meaning conventionally used in the colorimetry art. That is, the "primaries" are not used as real colors, but are merely three functions of wave length that have certain features found to be convenient for the designation of color.
Photoelectric colorimeters are known wherein light from a light source is directed through an optic system against a color sample, the light being reflected by the color sample toward three photoelectric detecting devices. In front of each photoelectric detecting device is positioned a primary filter to conform the spectral sensitivity of the respective photoelectric detecting devices as close as possible to the respective primaries or tristimulus values. A measuring device is connected to the three photoelectric detecting devices and reads or measures the amounts of the respective primaries or tristimulus values in the light reflected from the color sample.
It has however been found that although it is theoretically possible to design primary filters which correspond exactly to the ideal, it is in fact not possible to manufacture primary filters which have transmission factors corresponding to the ideal. That is, the measurements of the amounts of primaries or tristimulus values in a given color sample have an inherent error therein due to differences between actual and theoretical transmission factors of the primary filters.
Past attempts to correct this error have involved attempts to alter the transmission factor characteristics of the primary filters by forming the primary filters by a plurality of superimposed colored plates. However, since the spectral characteristics of the colored plates depend upon the components of the materials employed therein, normally glass, it is generally impossible to exactly match the theoretical transmission factors. It has been found to be virtually impossible to accurately duplicate the theoretical transmission values over the complete wave length range of the primaries or tristimulus values. Further, by increasing the number of plates, the amount of light received or passed through the primary filter is undesirably decreased. Further, it will be apparent that attempts to fabricate primary filters by carefully superimposing a plurality of plates in an attempt to match theoretical transmission factors is a time consuming and thus expensive operation.